1. Field of the Invention
An object of the present invention is to provide scintillation device that can be used to measure attenuation by emission tomography. It can be applied more particularly in the medical field or in association with a gamma camera used in a standard way, and provides for more precise knowledge of the internal structures revealed by an emission tomography examination carried out by means of this gamma camera.
2. Description of the Prior Art
An emission tomography examination carried out by means of a gamma camera comprises the injection, into a body to be examined, of a radioactive marker. In the medical field, the marker, for example Technetium, is injected, in the form of a dilution, into the patient's body. While moving in his body, the radioactive marker produces a measurable radioactive phenomenon. The radioactive phenomenon essentially comprises the emission of gamma rays.
To measure this radioactive phenomenon, a gamma camera or scintillation camera provided with a plane scintillator crystal is used. A gamma camera essentially has a stand to support a detector surmounted by the scintillator crystal. The gamma camera also has a control and processing panel. The detector comprises an arrangement of photomultiplier tubes to produce electrical signals corresponding to detected scintillations. The scintillator absorbs the gamma radioactive radiation by photoelectrical effect. It transmits light radiation detected downline by the array of the photomultiplier tubes of the detector. The tubes are associated with computing means. These computing means can be used to determine the coordinates of a locus of interaction of the gamma radiation in the scintillator.
Since the radioactive emissions in the body are omnidirectional, the only way to do this localization efficiently is to interpose a collimator between the body and the scintillator. This collimator lets through radioactive radiation only in a chosen direction.
With examinations such as these, it is possible to produce projected images. If the gamma camera is made to rotate about the body of the patient while the radioactive phenomenon occurs (during an examination period of about half an hour), it is possible to acquire a certain number of parallel type projections with which section images can be reconstructed according to tomography methods of a known type. The projections are of a parallel type because the collimator lets through rays in only one direction perpendicular to its plane.
The above acquisition mode has, however, one drawback: before exciting the scintillator, the gamma rays emitted by the internal structures of the body have to cross other regions of the body and thereby undergo a corresponding attenuation. This disturbs the acquisition of the data and the exactness and precision of the data.
Many attempts have been made to take this attenuation into account without actually measuring it. However, the methods proposed for this purpose have proved to be of little benefit and, to date, the real measurement of the attenuation by transmission tomography appears to be the only approach that can be envisaged.
It is possible, for example, to consider measuring radiological attenuation by using the measurements of tomodensitometry proposed in modern tomodensitometers using X-rays.
However, this approach has two drawbacks. Firstly, the patient has to be moved from one machine to another. It can never be certain that he will take up the same position in the second machine as in the first one. It becomes difficult to compare or transpose the measurements. Moreover, the energies of the radiation used in each case differ from each other. It is X-radiation that is used in tomodensitometry and gamma radiation in the scintillation device, and the coefficients of attenuation measured all depend on the energy of the radiation absorbed.
One method of measuring the attenuation by transmission tomography, using a rotational gamma camera provided with its collimator and a large external radioactive plane source, has been tested with a certain degree of success.
However, there are several serious drawbacks that restrict the use of this method in practice. Firstly, the additional acquisition time (acquisition for the measurement of the attenuation by transmission) is too long. Indeed, it lasts about 30 minutes. Furthermore, it is difficult to prepare a plane radioactive source in the sense that it is never possible to be assured of the homogeneity of the emission of the different parts of the source. A plane radioactive source is also difficult to manipulate because of its weight. Finally, and above all, it gives rise to a permanent irradiation that is unacceptable to those persons who have to use it. A technique of this kind has, for example, been described in the article by Malko J. A. & al., "SPECT Liver Imaging using an Iterative Correction Algorithm and an External Flood Source" in Journal of Nuclear Medecine, 27:701-705, 1986.
Another method has been proposed associating a point gamma source with a gamma camera without collimator. It has been described by Deconinck F. & al. in "Computerized Transmission Densitography and Tomography with a Gamma Camera", INSERM conference, INSERM 1979, vol. 88, pp. 245-256. In this method, a point source is placed at two meters from a gamma camera in a horizontal plane. The patient is brought closer in a rotating chair between this source and this gamma camera. The chair rotates about a vertical axis. The distance of two meters is a minimum distance to allow the gamma radiation emitted by the point source to be considered as a radiation parallel to the position of the irradiated body. The different projections are acquired by making the chair rotate.
This last-named technique has two drawbacks. Firstly, the distance at which the source is located is such that it becomes impossible to envisage the production of a structure that is rigid enough to enable the gamma camera (which is heavy) and the source to be kept in a relationship of correspondence when this assembly is rotating about the body, without its becoming necessary to undergo vibrations. Furthermore, the volumes needed for medical examination rooms would be off standards. Besides, this impossibility dictates a situation where it is the patient who has to be made to move in relation to the camera, and this is impossible in certain cases, especially when the patients are physically weak and are incapable of taking a vertical or seated position. It is necessary to take into account a situation where the patient slumps down in the course of time. This causes the interpenetration of the sections acquired and thus falsifies their reconstruction.
The disadvantage of bringing the source closer to the gamma camera to thus constitute a structure that can be manipulated around the patient is that it means losing the parallel character of the radiation which consequently bars the use of known algorithms for the reconstruction of the attenuation images acquired.
It is an object of the invention to overcome these drawbacks by using, a point source positioned in correspondence with and in the vicinity of the gamma camera. In practice, the proximity is less than one meter: in a preferred example, it is 70 cm.
Then, to resolve the problems of conical geometry prompted by this proximity, the following operations are performed: a weighting operation corresponding to an arrangement of all the conical projections thus obtained in a set of parallel projections, and a log standardization by the intensity of the source. Under these conditions, it is possible to have a nearby point source available, the efficiency of which is further increased because of its greater proximity to the body. In this case, then, no collimator is used, making the gamma camera 300 to 500 times more sensitive than it would be if it had a collimator. Under these conditions, it is possible to reduce the acquisition times for the measurement of the attenuation in transmission to about one minute: this is the time needed to acquire 64 projections for a duration of one second for each projection.
The equipment that can be used is then a standard type of equipment: the same as the one used for transmission tomography (i.e. tomography that involves revealing the presence of markers in the body). In the invention, given the fact that a standard type of equipment is used, the collimator which has been removed is judiciously replaced by a leaded frame of the same weight, shuttered if necessary by a plastic sheet that is transparent to gamma rays. This makes it possible to preserve the equilibrium of the standard gamma camera used. If the gamma camera is round, the frame is round. If the gamma camera is rectangular, the frame is rectangular. The leaded frame fulfills above all the role of a field shutter reducing the detection area to the useful zone and then enabling the counting rate of the gamma camera to be used in the revealing of the effective phenomena.
With the device of the invention, a high image resolution is achieved, related to the instrinsic quality of resolution of the detector without collimation. This is also due to the enlarging effect of the conical geometry related to the selected proximity of the gamma ray source. Thus, despite the aberrations related to this three-dimensional conical geometry, a better result is obtained.
Furthermore, the use of a point source has been turned to advantage to make a radioactive source, the use of which will be, firstly, easy and without danger for the operators and, secondly, can be implemented without raising the other problems encountered in the prior art. Indeed the only sources that can be envisaged are liquid sources. The fact of shifting the source dictates the need to manage the position of an air bubble that shifts depending on whether the source is placed beneath the gamma camera or above it, and on whether it is turned upside down or not. In the invention, the point source is made by means of a syringe that is filled and then partially emptied, in plunging its end into a jar, and in expelling the air bubble by reinjection into the jar. In this way, the syringe no longer has any air bubble. The position of the center of gravity of the radioactive source is then kept fixed in relation to the syringe, irrespectively of the orientation of this syringe (whether it is oriented upwards or downwards).